Spectacles of different uses are known, among which sunglasses are used to protect the wearer's eyes from being damaged by intense sun light in outdoors activity or other intense light. Conventionally, some of the lenses of the sunglasses are made of PMMA (polymethyl methacrylate) acrylic material. An anti-ultraviolet agent may be mixed within the PMMA acrylic material, The PMMA acrylic material is molded, for example injection molding, to form a "primary lens" to be further processed. The "primary lens" may then be strengthened by for example soaking in chemicals and high temperature treatment, to enhance for example surface hardness. Finally, a thin layer of for example Cr is coated in a vacuum environment on the strengthened lens to (1) reflect a portion of the incident light and (2) provide the lens with different colors.
The conventional sunglass lens, however, provides very limited variation in the decorative surface aesthetics. Thus, sunglass lens having hologram structure formed thereon was developed, such as U.S. Pat. No. 4,315,665 which discloses a composite optical structure comprising a substrate having surface variation defining a hologram on which a thin layer of uniform thickness made of a partially light reflecting material having a uniform thickness is coated and an overcoat provided on the partially light reflecting material. The structure disclosed in the U.S. patent requires a sandwich structure wherein a partially light reflecting layer, such as an aluminum layer, is sandwiched between two outside layers. This increases the manufacturing cost for it has to provide a uniform thickness of the partially light reflecting layer.
Also, if a dielectric material, such as SiO, having a diffractive index very close to the substrate material, such as PMMA, is coated on the surface variation of the substrate, then the 3-D image formed by the hologram may not be bright enough to be observed by other persons.
It is therefore desired to provide a spectacle lens structure which is capable to provide a 3-D holographic image without the above-mentioned problems encountered in the prior art.